This invention relates to a temperature control apparatus and method for producing an alloy coating on a running length of metal in a heat treating furnace, and more particularly to an improved apparatus producing a zinc iron alloy coating on a running length of steel strip by initially coating the strip with a layer of zinc or zinc containing a small amount of aluminum as in a hot dipped galvanizing operation and subsequently converting the coating to a zinc iron alloy in an electric induction heat treating furnace in which the temperature of the strip is controlled by sensing the radiation emissivity of the strip as it leaves various sections of the induction heat heating furnace.